1. Field of the Invention
This invention relates to an analog optical transmission system for transmitting a light signal.
2. Description of the Prior Art
An analog optical transmission system, having an optical fiber amplifier, for transmitting a light signal having an wavelength of 1550 nm band is known. FIG. 8 is a block diagram of such a prior art analog optical transmission system. A DFB (distributed feedback) laser 7 emits a laser light having a wavelength of 1550 nm band. The laser light is intensity-modulated by an analog signal modulated by AM-FDM (amplitude modulation with frequency division multiplex) or FM-FDM (frequency modulation with frequency division multiplex) as a laser light signal. The emitted laser light signal is amplified by an optical fiber amplifier 8 having an Er-doped optical fiber and a semiconductor laser for emitting laser light having a wavelength of 1480 nm for excitation of the Er-doped optical fiber. The amplified laser light signal is transmitted through a single mode optical fiber 9 having a length of 10 km. The transmitted laser light signal is received by an optical receiving unit 10. The optical receiving unit 10 converts the received laser light into an electric signal. Since the optical fiber amplifier 8 has a high gain at the 1550 nm band, it can be used as an booster amplifier. Therefore, this optical transmission system provides an optical distribution system for distributing video signals of more than a hundred of channels from a source to customers through all optical elements.
As mentioned, the intensity-modulation of the laser light signal emitted from the semiconductor laser also causes a frequency modulation of the emitted laser light signal. Moreover, the optical fiber amplifier 8 has a high gain at the 1550 nm band. However, the optical fiber amplifier 8 has a wavelength-dependency (frequency-dependency). Therefore, when the laser light signal subjected to frequency modulation as well as intensity modulation is transmitted through the optical fiber amplifier 8 having the frequency dependency, a secondary intermodulation component is developed by an interaction between the frequency modulated component and the frequency dependency in gain, so that the intensity modulated laser light signal is largely distorted. That is, in the prior art analog optical transmission system, there is a problem that the optical fiber amplifier 8 used for amplifying the laser light signal generates a large amount of distortion.